Objectives:
- To determine the pH of soil.
Introduction:
A soil analysis is a process by which elements such as P, K, Ca,
Mg, Na, S, Mn, Cu and Zn are chemically extracted from the soil and
measured for there “plant available” content within the soil sample.
Significance of Soil Analysis:
- It increases the knowledge of what nutrients are especially available in our soil.
- It reduces the environmental impacts due to soil amendments.
- It increases the efficiency of resource inputs such as fertilisers and water.
- It helps to predict the nutritional values needed for crop production.
- It helps to evaluate the fertility status of soils of a country or a state or a district.
Procedure for Taking Good Soil Samples:
- Determine the soil unit (or plot).
- Make a traverse over the soil unit (or plot).
- Clean the site (with spade) from where soil sample is to be collected.
- Insert the spade into soil.
- Standing on the opposite side, again insert the spade into soil.
- A lump of soil is removed.
- A pit of ‘V’ shape is formed. Its depth should be 0-6" or 0-9" or 0-12" (i.e., Depth of tillage).
- Take out the soil-slice
(like a bread - slice) of ½ inch thick from both the exposed
surface of the pit from top to bottom. This slice is also termed
furrow-slice. To collect the soil-slice spade may be used. Collect the
soil samples in a polyethylene bucket.
- Collect furrow-slices from
8-10 or sometimes 20-30 sites. Select the sites at random in a
zigzag (or criss-cross) manner. Distribute the sites throughout
the entire soil unit (plot). In lieu of spade auger may be used.
Do not take the prohibited samples and local problem soils.
- Furnish the following
information in two sheets of thick paper with the sample. One
sheet is folded and kept inside the bag. Another sheet is folded and
attached to the bag.
Hydrangeas...Blue or Pink?
Hydrangeas respond to the soil pH
where they are planted. They produce blue flowers in acid soil (pH 5.5
& <5.5), if the soil is highly acidic then the colour become the bluest of blue. Pink flowers are produced in alkaline soil (pH 7 & >7), if the soil highly alkaline then the colour of the flower become white such as ‘Lanarth White’.
Soil pH:
The soil pH reflects whether a soil is acidic, neutral, basic or alkaline.
The acidity, neutrality or alkalinity of a soil is measured in terms
of hydrogen ion activity of the soil water system. The negative
logarithm of the H ion activity is called pH and thus pH of a soil is a
measure of only the intensity of activity and not the amount of the
acid present. The pH range normally found in soils varies from 3 to 9.
Mathematically pH is represented as, log 1/H = - log H+
Apparatus:
pH Meter, Physical Balance, Beaker, Glass Rod, etc.
Principle of Potentiometric Method:
This method is essentially based on
the measurement of potential, developed across an indicator or the
glass electrode on account of the difference activity of H+ ions
in and out of the electrode, i.e., in the bathing solution. The
potential difference between the glass electrode and calomel electrode
is expressed in pH units.
Soil pH and Interpretation:
|
<5.0
|
5.5
|
6.0
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6.5-7.5
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7.5-8.5
|
>8.5
|
|
Strongly
Acid
|
Moderately Acid
|
Slightly
Acid
|
Neutral
|
Moderately
Alkaline
|
Strongly Alkaline
|
|
|
|
|
Best range for Most Crops
|
|
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pH Requirements of Some of the Commonly Grown Vegetables:
|
4.5 – 6.0
|
5.5 – 7.5
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6.0 – 7.0
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6.0 – 7.5
|
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Potato
|
Pumpkin
Cucumber
Cauliflower
Tomato
|
Onion
Lettuce
|
Cabbage
Spinach
Beans
|
Importance of pH:
- pH
determination is an indispensable means for characterizing soil from
the standard point of nutrient availability and physical condition,
structure, permeability, etc.
- It provides information on the potency of toxic substances present in the soil.
- It is indicative of the status of
microbial communities and its net effect on the neutralization of
organic residue and the immobilization of available nutrients.
- Ascertaining the soil pH provides the most rational basis for managing soil for selected agricultural crops.
- The pH measure of soil in water
and KCl systems provides information on the nature of charge discharge
on soil colloids which will have a far recharging effect on nutrient
measurement and reaction.
How to Reduce the Alkalinity of Soil?
Yards that contain high alkaline soil
with pH levels of 8 and above may find it hard to cultivate. Lowering a
soil’s alkalinity level is possible with the help of Ammonium Sulphate
and organic materials. This will lower the pH of soil. Similarly we
can reduce the soil’s acidity also, i.e., by adding lime to the highly
acidic soil. This will increase the pH of soil.
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